Editing Common Lisp (section)

====Defining generic functions and methods====
The macro <code>defgeneric</code> defines [[generic function]]s. Generic functions are a collection of [[Method (computer programming)|methods]].
The macro <code>defmethod</code> defines methods.

Methods can specialize their parameters over CLOS ''standard classes'', ''system classes'', ''structure classes'' or individual objects. For many types, there are corresponding ''system classes''.

When a generic function is called, [[multiple dispatch|multiple-dispatch]] will determine the effective method to use.

<syntaxhighlight lang="lisp">
 (defgeneric add (a b))
</syntaxhighlight>

<syntaxhighlight lang="lisp">
 (defmethod add ((a number) (b number))
   (+ a b))
</syntaxhighlight>

<syntaxhighlight lang="lisp">
 (defmethod add ((a vector) (b number))
   (map 'vector (lambda (n) (+ n b)) a))
</syntaxhighlight>

<syntaxhighlight lang="lisp">
 (defmethod add ((a vector) (b vector))
   (map 'vector #'+ a b))
</syntaxhighlight>
<syntaxhighlight lang="lisp">
(defmethod add ((a string) (b string))
  (concatenate 'string a b))
</syntaxhighlight>
<syntaxhighlight lang="lisp">
 (add 2 3)                   ; returns 5
 (add #(1 2 3 4) 7)          ; returns #(8 9 10 11)
 (add #(1 2 3 4) #(4 3 2 1)) ; returns #(5 5 5 5)
 (add "COMMON " "LISP")      ; returns "COMMON LISP"
</syntaxhighlight>

Generic Functions are also a [[First-class citizen|first class data type]]. There are many more features to Generic Functions and Methods than described above.